Brush holder assemblies in dynamoelectric machines are sometimes attached to a plate. Each of the brush holder assemblies accommodates a brush that may be electrically connected to a flexible conductor. The flexible conductor connected to the brush is routed through a hole or slot in a brush holder of the brush holder assembly. The end of the flexible conductor that is not connected to the brush may be electrically connected to a rigid conductor that sealably extends through a housing of the machine. Preventing the flexible and the rigid conductors from inadvertently shorting out against other conductive components of the dynamoelectric machine that carry alternate electrical potentials than that which is carried by the flexible or rigid conductors, is necessary for the proper operation of the dynamoelectric machine.
Brush holder assemblies typically also include a biasing spring for urging the brushes toward a commutator. The biasing force created by such springs can vary depending upon how the spring is seated against a surface of the brush, for example. Such variations in spring force can have a detrimental affect on the durability of the machine as well as have a detrimental affect on the audible and electrical noise emitted from the machine. It may, therefore, be desirable to minimize variations in brush spring force.
Mechanisms that attach the brush holders to the plates of dynamoelectric machines can also affect the brush spring force. In machines, in which the spring is compressed between the brush and the brush holder, variability in the positional attachment of the brush holder to the plate can affect the contact force of the brush against the commutator. It may therefore be advantageous to minimize the number of features involved in attaching the brush holder to the metal plate.
Another factor that affects not only the durability of the brush holder assembly but the complete dynamoelectric machine is the temperature at which the machine operates. The current that flows through the flexible connectors, brushes, commutator and windings of the machine can influence the operational temperature of the machine with higher currents causing higher operational temperatures. The amount of current that flows is affected by operational conditions of the dynamoelectric machine such as mechanical loading and friction, for example, and is therefore an unavoidable condition, which should be accommodated. Durability failures that result from over temperature operation can require replacement of the entire machine at a considerably higher cost than if only the brush holder assembly required replacement.
Over temperature operation can also result in fires. Some bush holder assemblies use the flexible conductors as fusible links, however, the flexible nature of such conductors may cause them to have a variation in current draw at which the open circuit occurs, resulting in premature failures of the conductors on some devices while permitting overheating on others. It may therefore be desirable to have a failure include an opening of the circuit at a more controlled current level than that which is available with the flexible conductors and to have the brush holder assemblies fail prior to the failure of the complete dynamoelectric machine.
Therefore, the art of brush holder assemblies is in need of simpler and less costly ways to prevent internal electrical short circuits, more reliable spring biasing forces, and integration of fusible links with tight control of overload currents.